They're both similar. Swan is smaller, closer to the walkway to DHS, and (somewhat) closer to BW. Dolphin is bigger, and closer to the YC/BC side. They both share the dock for the FriendShips to Epcot, BW and DHS, but in terms of buses I believe Swan gets picked up first.
Also, in terms of rates, I believe the Swan's are somewhat higher.
All things being equal, I'd choose the Swan. The Dolphin is nice, but at least to me it feels "too big".